Yeast flocculation: lectin synthesis and activation

Yeast flocculation involves binding of surface lectins to carbohydrate receptors on neighbouring cell walls. Brewing strains of Saccharomyces cerevisiae normally become flocculent in the stationary phase of growth. This paper presents evidence that lectins are synthesized in exponential phase, inserted into the cell wall, and activated later at the time of flocculation onset. Cycloheximide failed to prevent flocculation unless it was added in early growth; with later additions progressively larger degrees of flocculation occurred. Flocculation onset was delayed by cycloheximide but was otherwise cycloheximide insensitive. Preflocculent cells could be artificially activated to full flocculation by heat. Artificial activation of samples from growing yeast cultures confirmed the progressive synthesis of lectins throughout exponential growth. Pronase E treatment of whole cells prior to heating prevented any activation of flocculation. It was concluded that lectins were synthesized continuously from an early stage of growth and rapidly inserted into the cell wall (accessible by pronase E), where they remained inactive for up to 14 h, before being activated at flocculation onset by an as-yet unknown mechanism. It was found that lectin synthesis and activation occurred in all brewing strains tested.